Chronic Obstructive pulmonary disease (COPD) is characterized by excessive production of nitric oxide (NO) and hypoxia. The mechanisms underlying the progressive decline in lung function characteristic of these disorders is not fully understood. Recent studies have suggested that nitric oxide contributes to death of alveolar epithelial cells and fibroblasts. To date, no studies have examined the relationship of NO and hypoxia (10 torr) with respect to cell death. We demonstrate that cell death at a given concentration of NO is significantly higher under hypoxia compared to normoxia. We also found that cells transfected with the mitochondrial anti-apoptotic protein Bcl-xL prevent cell death by maintaining mitochondrial integrity, suggesting a key role for mitochondria in determining the relationship between NO and oxygen. Previous studies on mitochondria show that NO competes with oxygen for binding to cytochrome c oxidase. We hypothesize that hypoxia would lower the threshold of NO required to inhibit cytochrome c oxidase resulting in loss of mitochondrial membrane potential and subsequent cell death. In the present grant using NO donors with long half-lives (t1/2-24 hr), we propose to examine the mechanisms within the mitochondria by which NO-induces cell death in Rat 1a fibroblasts and mouse lung epithelial cells under hypoxia. We will examine the mechanisms by which hypoxia?s enhances nitric-oxide cell death.